The all-time British record is 38.5 degrees Celsius set in August 2003 near Faversham in Kent, south east England, while the highest July temperature was 36.7C in 2015 at Heathrow airport.

A Met Office spokesman said Thursday was expected to be the hottest day of the year so far.

"There is greater potential for both the July and the other (all-time) record to go tomorrow," he said, noting that the highest temperatures are expected in the southeast - in Kent, Essex, Suffolk and Cambridgeshire.

"It’s going to depend on the development of thunderstorms ... If the thunderstorms develop early that’s going to prevent some of the heat from the sun getting to the ground which will curb the rise of temperatures," he said.

Britain's heatwave has turned green grass brown, triggered an impending hose pipe ban in the northwest of England, and prompted a warning from the government for people to keep out of the sun. Farmers have also warned of possible food shortages later this year.